Accreted oceanic materials in Japan

Abstract

The Phanerozoic &cum-Pacific erogenic belts contain numerous ocean-derived materials accreted through plate converging processes. Japanese Islands, in particular, display various kinds of oceanic materials of different origins including fragments of seamounts, oceanic reef limestone, MORB-like rocks and oceanic mantle, and pelagic sediments. The compilation of these rocks in many subduction complexes of Late Permian to the present, led to following conclusions.Accretion processes work effectively only for materials primarily composing the upper portion of subducting oceanic crust, i.e. Layer 1 and Layer 2. Many fragments of seamount with alkali basalt (600), hot-spot seamount (26), oceanic reef limestone (291), MORB-like basalt (200), and numerous cherts (more than 1000) are recognized as ancient oceanic materials accreted to the Japanese Islands. However, gabbros and mantle materials of Layer 3 and lower parts of the oceanic lithosphere, scarcely occur in subduction-accretion complexes except for a few examples of back-arc basin or fore-arc origin. Accretion occurs episodically. In Southwest Japan, oceanic materials were accreted intermittently in (a) end-Permian, (b) Middle-Late Jurassic, (c) Late Cretaceous times, (d) at ea. 50 Ma, and (e) in Miocene times, while in Northeast Japan and Hokkaido this ocwrred in (b) Middle-Late Jurassic, (cf Late Cretaceous, and (f) Early Cretaceous times.In wntrast to the general belief on accretion of younger oceanic plates, the majority of Japanese subduction-accretion complexes were formed during the subduction of plates, up to 160 Ma old. The accretionary events in end-Permian and Middle-Late Jurassic times coincide with northward collision of ancient island arcs, oceanic rises or seamount chains (of hot-spot origin) with the Asian continent. Accretion relevant to subduction of older plates may be controlled by the collision-subduction process of these topographic reliefs on an oceanic plate. In addition, the chronological coincidence with the continent collision-amalgamation between the Sino-Korean and Siberian platforms and between the Sino-Korean and Yangtze blocks, also implies collision-induced voluminous supply of elastics from back-arc regions and its contribution to the formation of huge accretionary complexes.Accreted fragments of ancient seamounts are much smaller than the average size of modem seamounts. This impties that most parts of a colliding seamount are not accreted but subducted together with the underlying oceanic crust to much deeper levels.With respect to the metamorphic grades for Japanese subduction complexes, oceanic materials have less than 1 vol.% in the zeolite facies, 15-20% in the prehnite-pumpellyite metagraywacke facies, and ca. 30% in the greenschist/glaucophane schist facies and albite-epidote amphibolite facies. This relationship indicates that the major process for landward accretion of oceanic materials is not off-scraping or sedimentary mixing at the trench, but underplating (subcretion) at much deeper levels of a subduction zone.

Accreted seamounts in North Tianshan, NW China: Implications for the evolution of the Central Asian Orogenic Belt

“…This ophiolitic mélange composition is similar to volcano-sedimentary sequences the accreted Paleogene ocean island sequences which are exposed in the Azuero Peninsula, west Panama (Buchs et al, 2011). Indeed, the association of basalt, breccia and limestone has been noted in many subduction-accretion complexes (e.g., Isozaki et al, 1990;Safonova and Santosh, 2014). Thus, we infer in conjunction with the geochemical signatures of the basalts, that the Bayingou ophiolitic mélange represents the remnants of a series of seamounts that are likely to have been oceanic islands or small plateaus.…”

Future accreted terranes: a compilation of island arcs, oceanic plateaus, submarine ridges, seamounts, and continental fragments

“…The enigmatic Siletz terrane of northern California and Oregon is composed of volcanics with OIB signatures that have been variously interpreted as a hot spot track, slab window, and mid-ocean ridge (Schmandt and Humphreys, 2011;McCrory and Wilson, 2013). Examples of accreted seamounts, identified primarily by their OIB signature, are the alkali basaltic units found in Japan (Isozaki et al, 1990). Typical seamountderived terranes include thin-skinned units of radiolarian cherts, limestones, serpentinized peridotites, layered gabbros, and alkali basalts that are on the order of hundreds of meters thick (Geldmacher et al, 2008;Buchs et al, 2009).…”

Heat flow in the Southwest Japan Arc and its implication for thermal processes under arcs

“…The southwest Japan arc is composed of subduction complexes from the Permian to the present day [Isozaki et al, 1990]. At the trench accretion of sediments is presently active, and the accretionary prism is well developed.…”